Regenerative Fuel Cells

ABSTRACT

There is provided a regenerative fuel cell capable of operating in a power delivery mode in and in an energy storage mode. The cell may comprise a reversible hydrogen gas anode, in an anode compartment, a reversible cathode in a cathode compartment, and a membrane separating the anode compartment from the cathode compartment, which membrane is capable of selectively passing protons. an additive may be provided in the cathode compartment.

TECHNICAL FIELD

The present invention relates generally to the field of regenerativefuel cell (RFC) technology. More specifically, the invention relates tothe use of additives in RFC technology.

BACKGROUND ART

Regenerative fuel cells, and the methods by which they are able to storeand deliver electricity, have been known for many years. They areelectrochemical apparatus for energy storage and power delivery. In thepower delivery phase, electrochemically active species are supplied toelectrodes, where they react electrochemically to produceelectrochemical power. In a storage phase, electrical power is used toregenerate the electrochemically active species, which are stored.

Because the electrochemically active species can be stored separatelyfrom the electrode compartments and supplied when required, thegenerating capacity of this equipment can be quite large and scalable.

The electrochemical reactions take place on either side of an iontransport system (such as a membrane) with charge carriers either beingtransported or exchanged by the membrane.

The fundamental electrochemical process in these regenerative fuel cell(RFC) systems can be described by a simple redox equation where theaction proceeds in one direction in the energy storage mode of thesystem and in the opposite direction during the power delivery mode ofthe system and the term “redox” defines reactions in which a reductionand a complementary oxidation occur together.

However the implementation of these systems in practical applicationshas encountered major limitations, despite what appears to be a simpleelectrochemical process. Practical problems have included the use ofhazardous materials, poor efficiencies, system size, plugging andclogging of the flow of the electrolytes, crossover of the species, gasformation and, especially, the cost of materials and the cost ofequipment. These have prevented RFCs from being employed widely inindustry.

There is a wide range of potential applications for energy storagetechnologies. Most renewable energy technologies cannot easily adjusttheir power output to meet fluctuating demand and therefore energystorage is important in enabling low carbon/renewable energy sources tobe implemented in practice. Energy storage technologies may also be usedas a remote power source, to ensure constant power supply and qualityand may also be used to reduce the cost of electricity by storing energywhen electricity is cheap and distributing the stored energy at peaktimes.

One of the disadvantages faced by all regenerative fuel cells that havea metal redox couple is that hydrogen and/or oxygen co-evolution mayoccur when the metal ions are electrochemically reduced.

A liquid/gas fuel cell which employs hydrogen gas and liquid bromineelectrolyte has been investigated by Livshits et al. (ElectrochemistryCommunications, 2006, vol. 8(8), 1358-62). Later, the hydrogen-brominefuel cell was adapted into a RFC by EnStorage Flow Systems. Althoughthis system has demonstrated a high discharge output power, there are anumber of drawbacks to using this system including a low catalyststability and bromine gas evolution during operation. More recentlyWO2011/089518 has proposed a hydrogen-bromine regenerative fuel cell andalso mentions using a hydrogen-iron redox system. However, due to thelow standard electrochemical potential of the iron II/III redox couple(0.77 V vs SHE), the average working voltage of such a hydrogen-ironsystem at discharge will be even lower (indeed it is the lowest amongknown regenerative fuel cells) which is a significant disadvantage forpractical redox battery applications. Another liquid/gas RFC which hasbeen investigated is the vanadium/air RFC (Hosseiny, S. S., et al,Electrochemistry Communications, 2011, vol. 13, 751-754); however thissystem has low efficiency, low power density and poor rechargeability.

It is important to realise that regenerative fuel cells are distinctfrom standard fuel cells. Standard fuel cells consume fuel and cannormally only be run in a power delivery mode; they either cannot be runin an energy storage mode (in which power is stored) or, if they can,they can only do so in a highly inefficient way. Furthermore, reversingthe electrochemical reaction in a fuel cell can cause permanent damageto the catalyst. Standard fuel cells are optimised for operating in theenergy generating mode only while regenerative fuel cells are optimisedfor the combined power delivery mode and the energy storage mode. Thusonly electrochemical reactions that are readily reversible can be usedin a regenerative fuel cell, while in certain fuel cells (such as directalcohol, or direct borohydride fuel cells, or hydrogen/oxygen fuelcells) the reactions need not be reversible and indeed they are usuallynot. Because of these considerations, regenerative fuel cells willnormally use at least one different electrochemical reaction, ascompared to standard fuel cells, although where a fuel cell clearly useshalf cells that both use a readily reversible redox reaction, e.g. thehydrogen-I system disclosed in “Advancements in the Direct HydrogenRedox Fuel”, Electrochemical and Solid-State Letters, 11 (2) B11-B15(2008), such a system can be used both in fuel cells and regenerativefuel cells.

In addition the average operating voltage during discharge is important.A low voltage system will require either a higher number of cells inelectrical series to increase the voltage, or the design of bespokepower converters to deal with low voltage-high current systems, whichadds both complexity and cost to the system.

Therefore finding two redox couples for use in a regenerative fuel cellthat are reversible, soluble at practical concentrations (about 1 M orabove), have a suitable potential difference between the standardelectrode potentials (E^(⊖)/V) of the couples and overcome the problemsin the art is a challenging task.

WO2013104664 (A1) overcomes the above problems by providing a hydrogengas/dissolved metal ion regenerative fuel cell where the metal isselected from vanadium, cerium, manganese or their stable andelectrochemically reversible aqueous complexes. Vanadium, cerium andmanganese have relatively high electrochemical redox potentials:

Redox reaction Standard potential E₀ Ce⁴⁺ + e⁻ 

 Ce³⁺ 1.72 V Mn³⁺ + e⁻ 

 Mn²⁺ 1.54 V VO₂ ⁺ + 2H⁺ + e⁻ 

 VO₂ ²⁺ + H₂O 0.99 V

In particularly preferred embodiments, WO2013104664 (A1) provides ahydrogen gas/dissolved vanadium ion regenerative fuel cell. Theinventive regenerative fuel cells, and especially the hydrogen/vanadiumion system, at least partly overcome the problems of the all-vanadiumRFCs (VRBs) currently in use in that costs are significantly reduced byhalving the amount of expensive vanadium required. Furthermore,replacing large liquid electrolyte storage tanks with compressed gasstorage vessels for hydrogen dramatically reduces the amount of spacetaken up by the regenerative fuel cell, which further reduces costs.Other advantages include an increase in output power of the system dueto the lower overpotential of the hydrogen oxidation reaction. Thepreferred hydrogen/vanadium RFC provides a further advantage in thatexisting vanadium/vanadium RFC systems can be readily adapted to replacethe vanadium anode with a hydrogen anode, thereby reducing the capitalcost of installing the regenerative fuel cells of the present inventionby preventing the need to install an entire system. This retro-fittingaspect therefore overcomes a substantial draw-back of using new systemsfor those who have already invested considerable capital in existingtechnology and is an important aspect of the present invention.

Despite the problems overcome by the RFCs disclosed in WO2013104664 A1,the issues of poor efficiencies, system size, plugging and clogging ofthe flow of the electrolytes, ion exchange membrane fouling and highcapital cost, remain, preventing RFCs from being employed widely inindustry.

For example, Mn is an abundant and cheap metal with high redox potential(E^(⊖−)Mn(III)/Mn(II)=1.51 V) interesting for energy storageapplications. However, the implementation of Mn-based electrolyte inregenerative fuel cells has been elusive to this date due to undesiredprecipitation reactions. During cell charging Mn(III) is produced and asshown below in Equation 2, this ion spontaneously undergoesdisproportionation leading to precipitation of MnO₂ with a consequentdrop in cell capacity.

2Mn³⁺+2H₂O→Mn²⁺+MnO₂+4H⁺  Equation 1

Higher metal solubility in the liquid phase permits higher cell energydensity and reduces the frequency of plugging and clogging of the flowof the electrolytes and reduces ion exchange membrane fouling. In thiscontext, the present invention concerns liquid electrolytes containingorganic and/or inorganic additives in hydrogen (anode)/liquidelectrolyte (cathode) regenerative fuel cells leading to an improvedperformance. This has a dramatic positive impact on the cost of thesystem, efficiency and energy density of the cell unreported to thisdate.

Regenerative fuel cells can generally be distinguished from fuel cellsby their “plumbing”. A regenerative fuel cell has both conduits forsupplying fuel to the electrodes for the power delivery phase, and alsoconduits for conducting the spent fuel to a store so that it can beregenerated. Often the fuel will be in the form of electrolyte that isexhausted following a power delivery phase and in this case the conduitswill also be arranged to conduct exhausted (or spent) electrolyte to astore and supply it back to its half cell during an energy storage mode,e.g. by the use of appropriate pumps. In contrast, fuel cells are notset up to operate in energy storage mode to electrochemically replenishexhausted electrolyte. In the case of regenerative fuel cells having ahalf cell containing a gas electrode, a compressor is generally providedto compress gas generated during the energy storage mode to enable it tobe collected in a compressed gas storage tank for future power deliveryphases (although neither a compressor nor a storage tank is requiredwhere the electrode is an oxygen electrode since oxygen is freelyavailable from the atmosphere). In contrast a fuel cell will generallynot have such a compressor.

DISCLOSURE OF THE INVENTION

The present invention is defined in the accompanying claims.

The present invention relates to a regenerative fuel cell (RFC), that isto say an electrochemical apparatus configured for both energy storageand power delivery and has an anolyte compartment and a catholytecompartment separated by a membrane capable of selectively passingprotons, that is to say the membrane can transfer protons e.g., by beingselectively permeable to protons or is a proton exchange membrane.Specifically, the present invention relates the use of additives in theelectrolyte formulation to improve the system.

In accordance with standard terminology in the field of regenerativefuel cells, the terms “anode” and “cathode” are defined by the functionsof the electrodes in the power delivery mode. To avoid confusion, thesame terms are maintained to denote the same electrodes throughout thetwo modes of operation (power deliver and energy storage) of the RFC.The terms “anolyte” and “catholyte” will be used to denote theelectrolyte in contact with the “anode” and “cathode”.

In a power delivery mode, an electrochemically active species isoxidised at the anode and an electrochemically active species is reducedat the cathode to form reacted (or “spent”) species. In the energystorage mode, the electrochemical system is reversed and the “spent”cathode species is electrochemically oxidised at the cathode toregenerate the corresponding electrochemically active species. It willbe appreciated that the redox reaction at the hydrogen gas anode willnot produce any “spent” species in the power delivery mode as theelectrochemically active species (hydrogen gas) is transformed intoprotons that are dissolved in the electrolyte. Protons are selectivelypassed by the membrane separating the anode and cathode compartmentsfrom the anode side of the membrane to the cathode side of the membrane.The electrons produced during the oxidation of the hydrogen gas at theanode during the power delivery mode are collected by a currentcollector. However, any unreacted hydrogen gas may be transferred awayfrom the anode compartment by one or more conduits and returned to apressurised gas source vessel. In the energy storage mode, protons areselectively passed by the membrane separating the anode and cathodecompartments from the cathode side of the membrane to the anode side ofthe membrane and protons are reduced at the anode to regenerate thehydrogen gas, which forms the anode electrochemically active species.

Therefore, in accordance with an aspect of the present disclosure thereis provided a regenerative fuel cell that is capable of operating in apower delivery mode in which it generates electrical power by thereaction of an electrochemically active species at an anode and thereaction of a different electrochemically active species at a cathodeand that is also capable of operating in an energy storage mode in whichit consumes electrical power to generate said electrochemically activespecies, the cell comprising:

-   -   a reversible hydrogen gas anode, in an anode compartment;    -   a reversible cathode in a cathode compartment;    -   a membrane separating the anode compartment from the cathode        compartment, which membrane is capable of selectively passing        protons;    -   conduits configured to supply electrochemically active species        to the anode and to the cathode in said power delivery mode, and        to carry generated electrochemically active species away from        the anode and away from the cathode in said energy storage mode;        wherein the redox reaction at the anode is:

2H⁺+2e ⁻

H₂ (gas)

and the redox reaction at the cathode is one selected from:

V⁵⁺ +e ⁻

V⁴⁺,  (i)

Ce⁴⁺ +e ⁻

Ce³⁺, and  (ii)

Mn³⁺ +e ⁻

Mn²⁺  (iii)

-   -   a metal ion such as Al³⁺, TiO²⁺, Ti⁴⁺ or an organic material        such as a surfactant, chelating agent, a polymer or a dendrimer,        present as an additive.

For the cathode side of the regenerative fuel cell, theelectrochemically active species is present in liquid catholyte suppliedto the catholyte electrode compartment. In respect of the anode, theelectrochemically active species is hydrogen and the anolyte electrodeis a gas permeable electrode.

The electrochemically active species present in the catholyte may bestored in a first vessel ready for passing to the cathode compartment inthe power delivery mode. For the anode, the electrochemically activespecies is hydrogen gas, and will generally be in the form of apressurised gas source.

During the power delivery mode, the spent catholyte is collected in avessel, which may be the first vessel or a separate second vessel.During the energy storage mode, the spent electrolyte may be suppliedfrom the vessel to the cathode compartment, where the electrochemicallyactive species is regenerated. The first and second vessels (ifprovided) may be different compartments of a single container.

The regenerative fuel cell may additionally include a pump which allowsthe liquid catholyte to be circulated through the conduits between thestorage vessel and the cathode compartment. As described above, theregenerative fuel cell may include a compressor that allows the hydrogengas to be stored at pressure in a source vessel exterior to theelectrochemical cell. The regenerative fuel cell may also additionallyinclude a dryer which dries the hydrogen gas before it is stored in thesource vessel. The regenerative fuel cell may also be equipped with ahydrogen expander-generator to deliver electricity as a result ofcompressed gas expansion.

The electrochemical reactions may take place at a discrete anode andcathode or, particularly in the case of the gas anode, they willgenerally take place at the gas separation membrane/catalysed porous gaselectrode interface and so it may not always be easy to identify adiscrete anode and cathode and the main manifestations of the anode andthe cathode may simply be the anodic and cathodic current collectors,which facilitate the supply of electrons to an electrode from anexternal circuit and the removal of electrons from an electrode to theexternal circuit (in the power delivery mode, the anodic currentcollector will transfer electrons away from the anode to an externalcircuit, and the cathodic current collector will supply electrons fromthe external circuit to the cathode. In the energy storage mode, thiswill be reversed).

The anode is a porous gas electrode and the cathode may be a porous ornon-porous electrode, although porous electrodes are preferred. Examplesof suitable electrodes are well known in the art. Catalysed porouscarbon electrodes are particularly preferred in the present invention,for example catalysed carbon paper, cloth, felt or composite. The carbonmay be graphitic, amorphous, or have glassy structure. In a particularlypreferred embodiment of the invention, the anode is a catalysedelectrode and the cathode is a non-catalysed electrode. As the redoxreaction at the cathode does not usually require catalysis, having aregenerative fuel cell whereby only one of the electrodes is catalysedmay allow the production costs of the regenerative fuel cell to besignificantly reduced; it is possible, but not necessary, to use somenon-noble metal catalyst and this would also keep the cost down ascompared to the use of noble metal catalysts. The catalyst used in theanode may be of noble metals such as for example platinum, palladium,iridium, ruthenium, rhenium, rhodium, osmium or combinations thereof,including alloys for example a platinum/ruthenium alloy or binarycatalyst such as PtCo, PtNi, PtMo etc. or ternary catalyst PtRuMo,PtRuSn, PtRuW etc. or chalcogenides/oxides as RuSe, Pt—MoO_(x) etc. Somebinary/ternary or other than pure precious metal catalysts can be moretolerant to probable catalytic poisoning as results of catholyte speciescrossover. The amount of precious metal can be significantly reduced ascompared to normal catalysed fuel cell gas electrodes since RFCs are notrequired to operate at the same high power mode as conventional fuelcells.

The electrochemically active species present in the anode half cell ishydrogen gas. Therefore, the redox reaction which takes place at theanode is:

2H⁺+2e ⁻

H₂ (gas)

The hydrogen gas is stored externally to the anode compartment in acontainer, which may be a pressurised gas source vessel. The hydrogengas may be supplied to the anode compartment by one or more conduits inthe power delivery mode and may be carried away from the anodecompartment by one or more conduits in the energy storage mode.

The electrochemically active species present in the cathode half cellmay be one of V⁵⁺, Ce⁴⁺ or Mn³⁺, and the respective “spent” species areV⁴⁺, Ce³⁺ or Mn²⁺. Therefore the redox reaction which takes place at thecathode is selected from:

V⁵⁺ +e ⁻

V⁴⁺,  (i)

Ce⁴⁺ +e ⁻

Ce³⁺, and  (ii)

Mn³⁺ +e ⁻

Mn²⁺.  (iii)

In a preferred embodiment, the redox reaction which takes place at thecathode is:

Mn³⁺ +e ⁻

Mn²⁺

Additionally, the catholyte contains an additive, which helps preventundesired precipitation reactions. The additive can be an inorganicadditive or an organic additive. When the additive is organic, it can bea surfactant, polymer, or dendrimer. When it is inorganic it can be asource of a metal ion. Specifically, it can be a source of Al³⁺, TiO²⁺or Ti⁴⁺.

The Regenerative Fuel Cells of the present invention provide theadvantages/overcome the problems discussed above, most notably (a) thereduction of costs as compared to the all-vanadium RFCs (VRBs) currentlyin use by halving the amount of expensive metal ions, and especiallyvanadium, required, (b) a reduction in the amount of space taken up bythe regenerative fuel cell resulting from the replacement of largeliquid electrolyte storage tanks in VRBs with compressed gas storagevessels for hydrogen, (c) an increase in output power of the system dueto the lower overpotential of the hydrogen oxidation reaction, (d) inthe case the preferred hydrogen/vanadium RFC, it can be retro-fitted toexisting VRBs which reduces the capital investment needed to replaceVRBs and (e) reduced rates of precipitation of electroactive species,which in turn results in higher metal solubility in the liquid phasepermitting higher cell energy density and reduced frequency of pluggingand clogging of the flow of the electrolytes and reduced ion exchangemembrane fouling.

It will be appreciated that although the electrochemically activespecies present in the cathode half cell is referred to as a free catione.g. M^(n+), it may be present in the catholyte solution as any stablepositively charged complex, for example, an oxide complex, such as VO₂ ⁺and VO²⁺ or a complex formed with a surfactant, polymer or dendrimeradditive. The nature of the positively charged complex will depend onthe materials used to produce the catholyte solution. For example, whenthe electrochemically active species present in the cathode half cell isvanadium, the liquid catholyte may be prepared using tetravalentvanadium oxide (VO₂), vanadyl sulphate (VOSO₄) or pentavalent vanadiumoxide (V₂O₅), Ce^(III) ₂(SO₄)₃/Ce^(IV)(SO₄)₂ or trivalent ceriumcarbonate Ce^(III) ₂(CO₃)₃. When the electrochemically active speciespresent in the cathode half cell is manganese, the liquid catholyte maybe prepared using divalent manganese (MnSO₄) or divalent manganesecarbonate (MnCO₃). The electrolytes will generally be aqueous.

The electrochemically active species in the cathode half cell is presentin liquid electrolyte. Acidic electrolytes are well known in the art andany standard acidic electrolytes may be used in accordance with thepresent invention. Preferred electrolytes include sulphuric acid, whichmay be an aqueous solution of concentrated sulphuric acid,methanesulfonic acid (MSA) or trifluoromethanesulfonic acid (TFSA), ormixtures thereof, most preferably sulphuric acid. Due to the highelectrochemical potential of redox couples such Ce and Mn, the use oforganic acid electrolyte will be preferable in order to minimise oxygengeneration during energy storage mode (charging). The use of any otherstrong acid is not prohibited if the acid can form soluble metal cationsbut not reduce or oxidise the catholyte. The concentration of theelectrochemically active species in the catholyte determines the powerand energy density of the regenerative fuel cell. Therefore, theconcentration of electrochemically active species in the catholyte ispreferably at least 0.2 M, and more preferably greater than 0.5 M, e.g.greater than 1 M or greater than 1.5 M. The maximum practicalconcentration of the electrochemically active species will generally begoverned by its solubility in the electrolyte as precipitation from theelectrolyte becomes an increasing problem at higher concentrations, andthe presence of precipitated materials in the cell is preferably avoidedsince it interferes with the flow of the electrolyte and the functioningof the regenerative fuel cell in question.

The membrane separating the anode compartment from the cathodecompartment is a membrane capable of selectively passing protons(hydrogen ions), which means that the membrane may be a proton exchangemembrane or a membrane which is permeable to protons. Preferably, themembrane is a proton exchange membrane. Proton exchange membranes arewell known in the art, for example, the Nafion™ ion exchange membraneproduced by DuPont. Although the Nafion™ membrane has good protonconductivity and good chemical stability, it has a number ofdisadvantages including a high permeability to vanadium cations and highcost. Therefore, in other preferred embodiments, the membrane issubstantially impermeable to metal cations, for example vanadium, ceriumand manganese cations.

The additive may comprise one or more of: ammonium phosphate, potassiumphosphate, Ammonium Carbonate, Polyacrylic acid, Polyacrylamide,Polymaleic acid, Alanine, Glycine, SodiumPolyPhos., Sod.TriPolyPhos,Ammonium Sulphate, Potassium Sulphate, Poly Styrene S. Acid, Teric PE61,ICI, Teric BL8, ICI, Flocon-100, Calgon EL-5600, Briquest 3010-25,pHreedom, SHMP, K2SO4, K3PO4, KHSO4, Ethlyene diamine tetramethylenephosphonic acid (EDTMP), and CL-4000. These additives may findparticular use where redox reaction at the cathode is (i) V⁵⁺+e⁻

V⁴⁺.

Indeed, according to a further aspect there is provided a regenerativefuel cell capable of operating in a power delivery mode in which itgenerates electrical power by the reaction of electrochemically activespecies at an anode and a cathode and in an energy storage mode in whichit consumes electrical power to generate said electrochemically activespecies, the cell comprising:

-   -   a reversible hydrogen gas anode, in an anode compartment;    -   a reversible cathode in a cathode compartment;    -   a membrane separating the anode compartment from the cathode        compartment, which membrane is capable of selectively passing        protons;    -   conduits configured to supply electrochemically active species        to the anode and to the cathode in said power delivery mode, and        to carry generated electrochemically active species away from        the anode and away from the cathode in said energy storage mode;        wherein the redox reaction at the anode is:

2H⁺+2e ⁻

H₂ (gas);

the redox reaction at the cathode is:

V⁵⁺ +e ⁻

V⁴⁺,

and the cathode compartment comprises an additive. The additive maycomprise one or more of: hydrogen chloride, ammonium phosphate,potassium phosphate, another phosphate species, ammonium Carbonate,polyacrylic acid, polyacrylamide, polymaleic acid, alanine, glycine,sodiumPolyPhos., sod.TriPolyPhos, ammonium sulphate, potassium sulphate,polystyrene s. acid, Teric PE61, ICI, Teric BL8, ICI, Flocon-100, CalgonEL-5600, Briquest 3010-25, pHreedom, SHMP, K2SO4, K3PO4, KHSO4, Ethlyenediamine tetramethylene phosphonic acid (EDTMP), and CL-4000. Acorresponding method may also be provided. As the skilled person wouldrecognise, preferred or optional features applied to one aspect may alsobe applied to another.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic sectional view of a liquid/gas regenerative fuelcell of the invention (the terms “liquid” and “gas” denoting the phasesof the electrochemically active material supplied to the twoelectrodes).

FIG. 2 is a charge/discharge curve for a vanadium/hydrogen regenerativefuel cell, using 0.23M vanadium.

FIG. 3A shows charge/discharge curves for a vanadium/hydrogenregenerative fuel cell, using 0.23M vanadium at variable currentlydensities.

FIG. 3B shows charge/discharge curves for a vanadium/hydrogenregenerative fuel cell, using 0.23M vanadium at variable flow rates

FIG. 4A shows a power density curve for a vanadium/hydrogen regenerativefuel cell, using 0.23M vanadium at a flow rate of 50 ml/min.

FIG. 4B shows a power density curve for a vanadium/hydrogen regenerativefuel cell, using 0.23M vanadium at a flow rate of 100 ml/min.

FIG. 5 shows a power density curve for a vanadium/hydrogen regenerativefuel cell, using 1.0 M vanadium at a flow rate of 100 ml/min; the graphshows a combined I-V curve (left) and a power curve (right).

FIG. 6 shows charge/discharge curves for a manganese/hydrogenregenerative fuel cell, using 0.2 M MnSO₄ in 5 M H₂SO₄ at a flow rate of50 ml/min and current density of 20 mA/cm², over four cycles.

FIG. 7 shows charge/discharge curves for a manganese/hydrogenregenerative fuel cell, using 0.2 M MnSO₄ in 5 M H₂SO₄ at a flow rate of50 ml/min, at variable current densities.

FIG. 8 shows charge/discharge curves for a manganese/hydrogenregenerative fuel cell, using 0.2 M MnSO₄ in 5 M H₂SO₄ and a currentdensity of 20 mA/cm² at variable flow rates.

FIG. 9 shows a power density curve for a manganese/hydrogen regenerativefuel cell, using 0.2 M MnSO₄ in 5 M H₂SO₄ at a flow rate of 50 ml/minand 0.001 A/s, 20 s/p, at varying states of charge.

FIG. 10 shows charge/discharge curves for a manganese/hydrogenregenerative fuel cell, using 0.2 M MnSO₄ with 0.2 M Ti(SO₄)₂ in 5 MH₂SO at a flow rate of 50 ml/min, over four cycles.

FIG. 11 shows charge/discharge curves for a manganese/hydrogenregenerative fuel cell, using 0.2 M MnSO₄ with 0.2 M Ti(SO₄)₂ in 5 MH₂SO₄ at a flow rate of 50 ml/min, at variable current densities.

FIG. 12 shows charge/discharge curves for a manganese/hydrogenregenerative fuel cell, using 0.2 M Ti(SO₄)₂ in 5 M H₂SO₄ at a flow rateof 50 ml/min and current density of 20 mA/cm² at variable manganeseconcentrations.

FIG. 13 shows power density curves for a manganese/hydrogen regenerativefuel cell using 0.2 M Ti(SO₄)₂ in 5 M H₂SO₄ at different manganeseconcentrations, at a flow rate of 50 ml/min at a scan rate of 0.001 A/swith 20 s/point.

FIG. 14 shows power density curves for a manganese/hydrogen regenerativefuel cell at different acid concentrations using 0.2 M MnSO₄ and 0.2 MTi(SO₄)₂ at a flow rate of 50 ml/min at a scan rate of 0.001 A/s with 20s/point.

FIG. 15 shows power density curves for a manganese/hydrogen regenerativefuel cell using 0.2 M MnSO₄ and 0.2 M Ti(SO₄)₂ in 5 M H₂SO₄ at a flowrate of 50 ml/min, at a scan rate of 0.001 A/s with 20 s/point atvarying states of charge.

FIG. 16 shows a comparsion of capacity retention of a manganese/hydrogenregenerative fuel cell with and without Ti(IV) present.

FIG. 17 shows charge and discharge curves for a manganese/hydrogenregenerative fuel cell at a current density of 20 mA/cm² using 0.2 MMnSO₄ in 3 M H₂SO₄.

FIG. 18 shows charge and discharge curves for a manganese/hydrogenregenerative fuel cell at a current density of at 20 mA/cm² using 0.2 MMnSO₄ and 0.2 M Ti(SO₄)₂ in 3 M H₂SO₄.

FIG. 19A shows Galvanostatic charge and discharge at 50, 75 and 100 mAcm⁻² of a H₂—V RFB using HCl as supporting electrolyte

FIG. 19B shows Coulombic efficiency, voltage efficiency, energyefficiency and specific capacity upon galvanostic charge and dischargeat 75 mA cm². Electrolyte 2.5M VOSO₄ in 6M HCl. Liquid flow rate 50 mLmin⁻¹. Gas flow rate: 100 mL min⁻¹.

FIG. 20 shows Galvanostatic charge and discharge at 50 mA cm⁻² of a H₂—VRFB using 0.2M NH₄H₂PO₄ as additive. Electrolyte 3M VOSO₄ in 3M H₂SO₄.Liquid flow rate 50 mL min⁻¹. Gas flow rate: 100 mL min⁻¹.

DESCRIPTION OF EMBODIMENTS AND EXAMPLES

FIG. 1 shows a schematic of a regenerative fuel cell in which theelectrochemically active materials used to generate power are (a)hydrogen gas (supplied to the anode) and (b) dissolved metallic ions ina liquid catholyte (supplied to the cathode).

In the power delivery mode, the liquid catholyte containing theelectrochemically active species M^(n+1) is pumped by a pump (11) from acompartment of a catholyte storage container (12A), through a conduit(12B) and into the catholyte compartment (9), where it is reduced at acathode (2) according to the half reaction:

M^(n+1) +e ⁻→M^(n)

The catholyte containing the spent electrolyte species M^(n) is thencarried away from the catholyte compartment through a second conduit (1)to the catholyte storage container (12A), where it is stored in acompartment separate from the fresh catholyte compartment. The catholytefurther comprises an additive.

The anode and at least part of the anolyte compartment (8) are formed bya porous gas flow electrode and hydrogen is supplied from a pressurisedgas source vessel (7) through a conduit (13), to the anode/anodecompartment (8), where the hydrogen is oxidised to protons (H⁺)according to the half reaction:

H₂→2H⁺+2e ⁻

and the current is collected by a current collector (4). A protonexchange membrane (3) separates the anolyte and catholyte compartments(8 and 9) and selectively passes the protons from the anolyte to thecatholyte side of the membrane (3) to balance the charge, therebycompleting the electrical circuit. Any unreacted hydrogen is carriedaway from the anolyte compartment (8) by a second conduit (5) andreturned to the pressurised gas source vessel (7).

In the energy storage mode, the system is reversed so that theelectrochemically active species M^(n) is pumped from the catholytestorage container (12A), through the conduit (1) to the catholytecompartment (9), where the spent electrolyte species M^(n) is oxidisedat the cathode (2) to form the electrochemically active species M^(n+1).The resulting regenerated electrolyte is transferred away from thecatholyte container (9) by the pump (11), through the second conduit(12B) to the catholyte storage container (12A). Meanwhile, protons atthe anolyte side of the proton exchange membrane (3) are catalyticallyreduced at the porous gas anode (4) to hydrogen gas; the hydrogen istransferred away from the porous anode (4) through the conduit (5) andoptionally compressed by the compressor (6) before being stored in thepressurised gas source vessel (7).

It will be appreciated that the species M^(n+1)/M^(n) may be any ofV⁵⁺/V⁴⁺, Ce⁴⁺/Ce³⁺, Mn³⁺/Mn²⁺, preferably Mn³⁺/Mn²⁺.

Example 1: Vanadium Cell

A 25 cm² active area regenerative vanadium fuel cell (RVFC) was producedand tested as set out below. Details of the particular components usedin the above cell are set out below: Serpentine flow channel plates wereCNC-machined from highly conductive polymer composite Electrophen (Bac2)purposely produced for fuel cell bipolar plate applications. Plastic endplates were cut from PTFE sheet and were sandwiched between aluminiumplates to guarantee even pressure distribution across the cell. Acommercially available HiSPEC™ M-200 Class MEAs (Johnson Matthey FuelCells) with 25 cm² of catalytic active area were used in all experiments[Johnson Matthey http://www.jmfuelcells.com/products/meas]. To minimiseiR losses and provide good electric conductivity gold plated coppercurrent collectors were inserted between end and flow channel plates.Finally, Viton rubber gaskets were placed between different cellcomponents to ensure the system be properly sealed during the operation.

Preparation of Vanadium Sulphate Solution

Vanadium catholyte solutions of 0.23 M and 1 M were prepared bydissolving corresponding amounts of vanadyl sulphate (Sigma-Aldrich) in5M concentrated sulphuric acid. A Masterflex easy-load peristaltic pumpand Masterflex Chem-Durance tubing were used to pump the vanadiumcatholyte through the cell.

Typically VRBs use vanadium concentrations in the order of 1.5M-2.0M.However, a 0.23M solution was chosen as it would suffice to give aninitial indication of the cells performance. Using a lower concentrationalso allows for shorter charge and discharge times at low currentdensities. This solution was used for all experiments, except whereindicated otherwise.

Charge and Discharge Cycle Standard Cycle

The following procedure details the standard steps taken when performinga charge/discharge cycle. The cathloyte and hydrogen flow rates remainedconstant throughout the procedure.

-   -   1. The system was discharged to a target voltage of 0.45 V using        the current density at which the cycle was to be performed. If        the system's state of charge (SOC) was below this target, the        system was charged to a point above the target SOC and then        discharged to 0.45 V.    -   2. The open circuit voltage (OCV) of the system was measured for        5 minutes.    -   3. The system was charged at the desired current density until        the upper voltage cut-off limit was reached.    -   4. The OCV of the system was measured for 5 minutes.    -   5. The system was discharged at the desired current density        until the lower voltage cut-off limit was reached.    -   6. The OCV of the system was measured for 5 minutes.

Cycle Between Set Capacities

-   -   1. The system was discharged to a target voltage of 0.45 V using        the current density at which the cycle was to be performed. If        the system's SOC was below this target the system was charged to        a point above the target SOC and then discharged to 0.45 V.    -   2. The time t (in seconds) required to reach the desired        capacity for a particular current is calculated using Equation        2.

$\begin{matrix}{{t = {\frac{n \cdot F \cdot C \cdot V}{I} = {{\frac{Q}{3.6 \cdot I}{t\lbrack s\rbrack}} = {3.6*\frac{{Capacity}\mspace{14mu}\left\lbrack {{mA}\; h} \right\rbrack}{{Current}\mspace{14mu}\lbrack A\rbrack}}}}}{{t\lbrack s\rbrack} = \frac{{{Capacity}\mspace{11mu}\left\lbrack {{mA}\; h} \right\rbrack} \times 3,{600\left\lbrack \frac{s}{hr} \right\rbrack} \times {1_{0}^{- 3}\left\lbrack \frac{A}{mA} \right\rbrack}}{i\lbrack A\rbrack}}} & {{Equation}\mspace{14mu} 2}\end{matrix}$

-   -   -   Where n—number of electrons, F—Faraday number, C—species            concentration [mole/L], V—total solution volume [L],            I—current [A], Q—capacity [mAh]

    -   3. The OCV of the system was measured for 5 minutes.

    -   4. The system was charged at the desired current density for the        time calculated in step 2 or until the upper voltage cut-off        limit was reached.

    -   5. The OCV of the system was measured for 5 minutes.

    -   6. The system was discharged at the desired current density for        the time calculated in step 2 or until the lower voltage cut-off        limit was reached.

    -   7. The OCV of the system was measured for 5 minutes.

The upper voltage cut-off of 1.7 V was selected due to the risk ofcorrosion to the carbon paper at voltages above 1.9 V (Joerissen et al,Journal of Power Sciences, 2004, vol. 127(1-2), 98-104) and excessiveoxygen evolution due to water oxidation. The lower voltage cut-off of0.45 V was selected due to the reduction of V(IV) ions to V(III) ions atvoltages below 0.35 V (Pourbaix diagram at pH 0).

System Testing Charge/Discharge Cycle at 0.23 M V(V)/V(IV)

A number of charge/discharge cycles were performed to analyse thebehaviour of the RVFC.

FIG. 2 shows the charge discharge behaviour at a current density of 8mA/cm². The system gives a smooth charge/discharge curve. The opencircuit potential (OCV) for the vanadium/hydrogen system can becalculated from the Nernst equation (Equation 3).

$\begin{matrix}{{E_{cell} = {E_{cell}^{o} - {\frac{RT}{F}\ln \frac{\left\lbrack {VO}^{2 +} \right\rbrack}{{\left\lbrack {VO}_{2}^{+} \right\rbrack \left\lbrack H^{+} \right\rbrack}P_{H_{2}}^{0.5}}}}}{E^{0} = {E_{cell}^{0} - {\frac{RT}{nF}{\ln \left( \frac{\left\lbrack {V({IV})} \right\rbrack \left\lbrack {H_{2}O} \right\rbrack}{{{\left\lbrack {V(V)} \right\rbrack \left\lbrack H^{+} \right\rbrack}\left\lbrack H_{2} \right\rbrack}^{\frac{1}{2}}} \right)}}}}} & (3)\end{matrix}$

The reactions in the power delivery mode are:

Anode: H₂(g)→2H⁺+2e ⁻.

Cathode: 2VO₂ ⁺+4H⁺+2e ⁻→2VO²⁺+2H₂O.

Total: H₂(g)+2VO₂ ⁺+2H⁺→2VO²⁺+2H₂O.

The OCV at the charged state for the cycle in FIG. 2 is 1.099 V. Usingthe Nernst equation, the percentage of V(V) species over V(IV) specieswas calculated as 99%. A higher concentration of V(V) species would beexpected at the charged state and the high OCV would indicate a veryhigh SOC. The OCV following discharge is 0.9213 V which corresponds to aV(V) concentration of 7%. The decreased V(V) concentration is expectedas the solution is discharged, and the low concentration would indicatea deep discharge.

Charge/Discharge Cycle at 0.23M V(V)/V(IV) at Different CurrentDensities

The system was charged to a target capacity of 264 mAh, as thisrepresented a 36% utilisation of the maximum theoretical capacity of 740mAh. The hydrogen and vanadium flowrates were maintained at 50 mL/minthroughout each cycle (FIG. 3A). From the results it was observed thatthe overpotential for both charge and discharge steps increased as thecurrent density increased. Overpotentials are due to losses associatedwith Ohmic resistance, charge transfer and mass transport phenomena. Atthe start of each charge/discharge step the losses are most likelypredominated by charge transfer processes while at the end it is mostlymass transport limitations that contribute to the overpotential.

Charge/Discharge Cycle at 0.23 M V(V)/V(IV) at Different Flow Rates

System parameters that affect the utilisation of redox species arecurrent density, voltage window and electrolyte flow rate. It has beenobserved that as the current density increases, utilisation ofelectrolyte decreases. Current density also determines the operatingpower of the system. There is a trade-off between electrolyteutilisation and power output. The effect of vanadium flow rate oncapacity utilisation was studied at a current density of 14 mA/cm² (FIG.3B). The maximum power point at 75% SOC was chosen as it is assumed tobe a good representative point for the system.

At a vanadium flow rate of 50 mL/min, the system had a capacityutilisation of 16.2% (see Table 1).

Decreasing the flow rate to 30 mL/min resulted in a decrease in capacityutilisation to 9.5%. However, increasing the flow rate to 70 mL/minresulted in an increase in the capacity utilisation to 31.6%. A 40%decrease in flow rate yielded a 41% decrease in the utilisation, while a40% increase yielded a 95% increase in the utilisation. This shows theimportance of mass transport and concentration polarisation losses tothe system. Further increasing the flow rate to 100 mL/min marginallyincreased the capacity utilisation to 32.7%, and achieved higherefficiencies.

TABLE 1 Capacity utilisation for a wide range of vanadium flow ratesCapacity Cycle Number [—] Utilisation [%] 1-30 mL/min 9.5 2 & 3-50mL/min 16.2 4-70 mL/min 31.6 5-100 mL/min 32.8Power Density for 0.23 M V(V)/V(IV) at a Flow Rate of 50 ml/min

A power curve was measured for the system at a vanadium flow rate of 50mL/min and at three states of charge (SOC) (FIG. 4A). For an SOC of 75%the system reached a maximum power point of 7.7 mW/cm² at a currentdensity of 14 mA/cm².

Power Density for 0.23 M V(V)/V(IV) at a Flow Rate of 100 ml/min

The power curve was measured for a flow rate of 100 mL/min and at threedifferent SOCs (FIG. 4B). At 100% SOC a maximum power point of 13.3mW/cm² was reached at a current density of 24 mA/cm². At an SOC of 75%the maximum power reached was 10.7 mW/cm² at a current density of 21mA/cm². This represents a 39% increase in the maximum power whencompared with the power reached at a flow rate of 50 mL/min. Operatingat higher vanadium flow rate improves system performance by reducingmass transport limitation and increasing capacity utilisation.

Power Density for 1.0 M V(V)/V(IV) at a Flow Rate of 100 ml/min

The concentration of vanadium was increased from 0.23 M to 1.0 M inorder to investigate the effect of increasing the vanadiumconcentration, an important consideration for large-scale systems. Atypical large-scale VRB system would have a vanadium concentration of1.5-2.0 M. Solubility and precipitation of V⁵⁺ becomes an issue atconcentrations above 2.0 M. A 1.0 M solution was chosen as it is asubstantial increase in the concentration from the original system andbrought it closer to the 1.5 M concentration that would be used atlarge-scale.

The power curve for the 1.0 M system was measured at a vanadium flowrate of 100 mL/min and at SOCs of 100%, 75% and 50% (FIG. 5). Themaximum power reached at 100% SOC was 54.3 mW/cm² at a current densityof 84 mA/cm². The maximum power point for an SOC of 75% was also at acurrent density of 84 mA/cm² with a specific power of 48.4 mW/cm². Ifthe maximum power point determined at 75% SOC for the 0.23 M system isscaled up linearly to 1.0 M, a power of 46.5 mW/cm² at a current densityof 91 mA/cm² would be achieved. This indicates a remarkably linearrelationship between maximum power and vanadium concentration. In thiswork we have found that our non-optimised RVFC has demonstratedsurprisingly high power density comparing to current commercial VRBtechnology. For example the discharge power density of common VRB systemwith 2 M vanadium solution and average working voltage of 1.3 V isaround 65 mW/cm² (Rychcik, M. and Skyllas-Kazacos, M., Journal of PowerSources, 1988, vol. 22, 56-67) or 32 mW/cm² for 1 M vanadium solutionwhich is 40% less than that of our non-optimised RVFC. This resultfurther reinforces the potential of an optimised RVFC system in terms ofpower output as compared with the current state-of-the-art all-vanadiumsystems.

An energy efficiency of 30.9% was achieved for the 1.0 M system, whenrun at a current density of 72 mA/cm² (just below the maximum powerpoint) and vanadium flow rate of 100 mL/min. This compares with anenergy efficiency of 33.5% for the 0.23 M system under similarconditions (Table 2). The 1.0 M solution has lower overall energyefficiency primarily due to its lower voltage efficiency; however, itdoes achieve higher coulombic efficiencies.

TABLE 2 Comparison of efficiencies for the 0.23M and 1.0M systems.Vanadium Efficiency Concentration Coulombic Voltage Energy [M] [%] [%][%] 0.23M 74.2 45.1 33.5 1.0M 86.2 35.9 30.9

Example 2: Manganese Cell

Unless stated otherwise, the experimental setup as described above wasused for manganese RFC experiments.

The methods to characterise manganese RFCs such as charge and dischargecycles and power curve measurements were also the same. Dischargecut-off voltages were 0.5 V while charging was stopped at 1.8 V.

Preparation of Manganese Sulphate Solution

Manganese catholyte solutions of 0.2 M and 0.5 M were prepared bydissolving corresponding amounts of manganese carbonate (Sigma-Aldrich)in concentrated sulphuric acid. Upon adding manganese carbonate, CO₂ isreleased facilitating metal solubility. Sulphuric acid concentration isadjusted to reach concentration of 3M or 5M. A Masterflex easy-loadperistaltic pump and Masterflex Chem-Durance tubing were used to pumpthe vanadium catholyte through the cell.

Electrochemical Reactions During Discharge:

Cathode: 2Mn³⁺+2e ⁻→2Mn²⁺ E^(⊖) _(Mn2+)=1.51 V

Anode: H₂→2H⁺+2e ⁻ E^(⊖) _(anode)=0V

Overall 2Mn³⁺+H₂→2H⁺±2Mn²⁺ E^(⊖) _(overall)=1.51 V

System Testing Standard Mn Electrolyte (No Additive): Cycle Efficiency

FIG. 6 shows charge/discharge curves for a manganese/hydrogenregenerative fuel cell, using 0.2 M MnSO₄ in 5 M H₂SO₄ at a flow rate of50 ml/min and current density of 20 mA/cm², over four cycles.

The Coulombic, voltage and energy efficiency for each cycle aresummarized below in Table 3.

TABLE 3 Comparison of Coulmbic, voltage and energy efficiency of 0.2Mmanganese system over four cycles. Coulombic efficiency Voltageefficiency Energy efficiency cycle 1 86% 83% 72% cycle 2 81% 78% 66%cycle 3 87% 66% 58% cycle 4 86% 74% 64%

Standard Mn Electrolyte (No Additive): Current Density

FIG. 7 shows charge/discharge curves for a manganese/hydrogenregenerative fuel cell, using 0.2 M MnSO₄ in 5 M H₂SO₄ at a flow rate of50 ml/min, at variable current densities.

The Coulombic, voltage and energy efficiency at different currentdensities are summarized below in Table 4.

TABLE 4 Comparison of Coulmbic, voltage and energy efficiency of 0.2Mmanganese system at different current densities. Coulombic EnergyCurrent density efficiency Voltage efficiency efficiency 10 mA/cm² 86.6%83.1% 71.9% 15 mA/cm² 85.0% 74.9% 63.6% 20 mA/cm² 90.9% 72.3% 65.8%

Standard Mn Electrolyte (No Additive): Flow Rate

FIG. 8 shows charge/discharge curves for a manganese/hydrogenregenerative fuel cell, using 0.2 M MnSO₄ in 5 M H₂SO₄ at a currentdensity of 20 mA/cm² at variable flow rates.

TABLE 5 Comparison of Coulmbic, voltage and energy efficiency of 0.2Mmanganese system at different flow rates. Energy Flow rate Coulombicefficiency Voltage efficiency efficiency 20 mL/min 77.7% 69.5% 54.0% 35mL/min 64.9% 73.0% 47.4% 50 mL/min 86.6% 83.0% 71.9%

Standard Mn Electrolyte (No Additive): Power Curve

FIG. 9 shows a power density curve for a manganese/hydrogen regenerativefuel cell, using 0.2 M MnSO₄ in 5 M H₂SO₄ at a flow rate of 50 ml/min ata scan rate of 0.001 A/s with 20 s/point, at varying states of charge.

Example 3: Manganese Cell with Additives

A similar experimental setup and series of experiments as above wereperformed to characterise regenerative manganese fuel cells withadditives.

Unless stated otherwise, the experimental setup as described above wasused for manganese RFC with additive experiments.

The methods to characterise manganese RFCs such as charge and dischargecycles and power curve measurements were also the same. Dischargecut-off voltages were 0.5 V while charging was stopped at 1.8 V.

Preparation of Manganese Sulphate/Titanium Sulphate Solution

Additive solutions of 0.2 M to 1.5 M were prepared by dissolving asuitable precursor in concentrated sulphuric acid. Manganese catholytesolutions of 0.2 M and 0.5 M were prepared by dissolving correspondingamounts of manganese carbonate (Sigma-Aldrich) in concentrated sulphuricacid. Upon adding manganese carbonate, CO₂ is released facilitatingmetal solubility. Sulphuric acid concentration is adjusted to reachconcentration of 3M or 5M.

A Masterflex easy-load peristaltic pump and Masterflex Chem-Durancetubing were used to pump the vanadium catholyte through the cell.

System Testing

Optimised Mn Electrolyte (with Ti(IV) Additive): Cycle Efficiency

FIG. 10 shows charge/discharge curves for a manganese/hydrogenregenerative fuel cell, using 0.2 M MnSO₄ with 0.2 M Ti(SO₄)₂ in 5 MH₂SO at a flow rate of 50 ml/min, over four cycles.

TABLE 6 Comparison of Coulmbic, voltage and energy efficiency of 0.2Mmanganese system with 0.2M Ti(IV) additive over four cycles. Coulombicefficiency Voltage efficiency Energy efficiency cycle 1 92.7% 72.3%67.0% cycle 2 96.9% 77.8% 75.4% cycle 3 98.1% 76.5% 75.1% cycle 4 96.9%76.2% 73.9%Optimised Mn Electrolyte (with Ti(IV) Additive): Current Density

FIG. 11 shows charge/discharge curves for a manganese/hydrogenregenerative fuel cell, using 0.2 M MnSO₄ with 0.2 M Ti(SO₄)₂ in 5 MH₂SO₄ at a flow rate of 50 ml/min, at variable current densities.

TABLE 7 Comparison of Coulmbic, voltage and energy efficiency of 0.2Mmanganese system with 0.2M Ti(IV) additive at different currentdensities. Coulombic Current density efficiency Voltage efficiencyEnergy efficiency 10 mA/cm² 97.3% 78.7% 76.6% 20 mA/cm² 88.6% 72.7%64.4%Optimised Mn Electrolyte (with Ti(IV) Additive): Manganese Concentration

FIG. 12 shows charge/discharge curves for a manganese/hydrogenregenerative fuel cell, using 0.2 M Ti(SO₄)₂ in 5 M H₂SO₄ at a flow rateof 50 ml/min and current density of 20 mA/cm² at variable manganeseconcentrations.

TABLE 8 Comparison of Coulmbic, voltage and energy efficiency of 0.2Mmanganese system with 0.2M Ti(IV) additive at different manganeseconcentrations. Coulombic Energy efficiency Voltage efficiencyefficiency 0.2M Mn(II)/Mn(III) 92.7% 72.3% 67.0% 0.5M Mn(II)/Mn(III)91.9% 72.7% 66.8%

FIG. 13 shows power density curves for a manganese/hydrogen regenerativefuel cell at different manganese concentrations using, 5 M H₂SO₄ at aflow rate of 50 ml/min at a scan rate of 0.001 A/s with 20 s/point.

Optimised Mn Electrolyte (with Ti(IV) Additive): Acid Concentration

FIG. 14 shows power density curves for a manganese/hydrogen regenerativefuel cell at different acid concentrations using 0.2 M Mn at a flow rateof 50 ml/min at a scan rate of 0.001 A/s with 20 s/point.

Optimised Mn Electrolyte (with Ti(IV) Additive): State of Charge

FIG. 15 shows power density curves for a manganese/hydrogen regenerativefuel cell at using 0.2 M Mn in 5 M H₂SO₄ at a flow rate of 50 ml/min ata scan rate of 0.001 A/s with 20 s/point at varying states of charge.

Effect of Ti⁴⁺ on Capacity Loss and Precipitation of MnO₂

FIG. 17 shows charge and discharge curves for a manganese/hydrogenregenerative fuel cell at a current density of 20 mA/cm² using 0.2 MMnSO₄ in 3 M H₂SO₄. Once fully charged, there was a pause of 10 minbefore starting the discharge. It can be seen that there was a largedrop in capacity between first and second cycles. MnO₂ precipitation wasevident, and believed to be the primary cause for loss in capacity.

FIG. 18 shows charge and discharge curves for a manganese/hydrogenregenerative fuel cell at a current density of at 20 mA/cm² using 0.2 MMnSO₄ and 0.2 M Ti(SO₄)₂ in 3 M H₂SO₄. Once fully charged, there was apause of 10 min before starting the discharge. Clearly, there is littleor no loss in capacity between cycles. There was no evidence ofprecipitation of solid, even keeping the liquid fully charged for 12hours. Clearly, the solution is stabilised by the presence of Ti⁴⁺, andcapacity is retained between cycles.

FIG. 16 shows a comparsion of capacity retention of a manganese/hydrogenregenerative fuel cell with 0.2 M MnSO₄, with and without 0.2 M Ti(SO₄)₂in 5 M H₂SO₄ over four cycles. Again the greatly improved capacityretention when titanium is present is clear. This is due to the improvedstability of the solution (i.e. reduction in MnO₂ precipitation).

Further Discussion

Vanadium-(V) state exhibits a rich aqueous chemistry with metal complexstructures that depends on the pH and vanadium concentration. Invanadium RFCs or Rebdox Flow Batteries (RFBs) VO₂ ⁺ is produced at thepositive electrode during battery charge. This species tends topolymerize and precipitate as V₂O₅ if its concentration is above 1.5M.This behaviour is enhanced as temperature rises above 40 C. Formation ofsolid precipitates leads to irreversible degradation of the RFC/RFB as aresult of capacity fade and cell clogging. Besides that, those vanadiumsolubility constraints lead to RFCs/RFBs with moderate-to-low energydensity (10-15 Wh/L⁻¹).

Recently, utilization of Cl⁻ ligands and HCl as supporting electrolyteas a strategy to achieve high V(V) concentration and stability has beenreported. While such catholyte formulation was successfully implementedin an all-vanadium RFB (liquid-liquid configuration), no reports onH2-Vanadium systems are available in the literature. Moreover, theimplementation of HCl as supporting electrolyte is recommended due tohalide crossover to the gas side which causes corrosion of the hydrogenevolution and oxidation catalyst (normally Pt). This effect has beenwidely reported for H2-Br systems as well as in the chlor-alkaliindustry. To our surprise, the utilization of an electrolyte made of2.5M VOSO4 in 6M HCl as positive electrolyte in a H2-Vanadium systemusing a Platinum Black—Carbon Paper Electrode anode (Fuel Cell Store,0.5 mgPt cm-2 loading), a Nafion 117 membrane and SGL Sigracet graphitefelt cathode yielded and excellent performance. FIG. 19A showsgalvanostic charge and discharge of the system using different currentdensities, 50, 75 and 100 mA cm⁻² respectively. The longer term chargeand discharge performance was studied over 10 cycles at 75 mA cm⁻²leading to energy efficiency on average above 90% and specific capacityof around 49 Wh L⁻¹ without considering the volume of gas produced (FIG.19B).

Another positive electrolyte formulation independently reported is thein-situ formation or addition of NH₄H₂PO₄ as additive. This moiety hasshown ability to stabilize V(V) for longer periods with highconcentration (3M). Such electrolyte formulation, has never beenexplored in a H₂-Vanadium concentration. FIG. 20 shows the firstsuccessful galvanostatic charge and discharge of such system using aPlatinum Black—Carbon Paper Electrode anode (Fuel Cell Store, 0.5 mgPtcm-2 loading), a Nafion 117 membrane and SGL Sigracet graphite feltcathode and a current density of 50 mA cm⁻². The electrolyte formulationwas 3M VOSO₄ in 3M H₂SO₄ containing 0.2M of NH₄H₂PO₄. FIG. 20illustrates in particular galvanostatic charge and discharge at 50 mAcm⁻² of a H₂—V RFB using 0.2M NH₄H₂PO₄ as additive; electrolyte 3M VOSO₄in 3M H₂SO₄; liquid flow rate 50 mL min⁻¹; Gas flow rate: 100 mL min⁻¹.

Alternative additives also considered beneficial, particularly in thecontext of Vanadium electrolytes, include other phosphate species suchas potassium phosphate and sodium hexametaphophate (SHMP) among others.Additives may include one or more of the following, amongst others:Ammonium Carbonate, Polyacrylic acid, Polyacrylamide, Polymaleic acid,Alanine, Glycine, SodiumPolyPhos., Sod.TriPolyPhos, Ammonium Sulphate,Potassium Sulphate, Poly Styrene S. Acid, Teric PE61, ICI, Teric BL8,ICI, Flocon-100, Calgon EL-5600, Briquest 3010-25, pHreedom, SHMP,K2SO4, K3PO4, KHSO4, Ethlyene diamine tetramethylene phosphonic acid(EDTMP), and CL-4000.

1. A regenerative fuel cell capable of operating in a power deliverymode in which it generates electrical power by the reaction ofelectrochemically active species at an anode and a cathode and in anenergy storage mode in which it consumes electrical power to generatesaid electrochemically active species, the cell comprising: a reversiblehydrogen gas anode, in an anode compartment; a reversible cathode in acathode compartment; a membrane separating the anode compartment fromthe cathode compartment, which membrane is capable of selectivelypassing protons; conduits configured to supply electrochemically activespecies to the anode and to the cathode in said power delivery mode, andto carry generated electrochemically active species away from the anodeand away from the cathode in said energy storage mode; wherein the redoxreaction at the anode is:2H⁺+2e ⁻

H₂ (gas); the redox reaction at the cathode is selected from:v ⁵⁺ +e ⁻

V⁴⁺,  (i)Ce⁴⁺ +e ⁻

Ce³⁺, and  (ii)Mn³⁺ +e ⁻

Mn²⁺; and  (iii) the cathode compartment comprises an additivecomprising at least one of: (i) Ti(IV), (ii) Al(III), (iii) asurfactant, (iv) a chelating agent (v) a polymer, and (vi) a dendrimer.2. The regenerative fuel cell of claim 1, wherein the redox reaction atthe cathode is:Mn³⁺ +e ⁻

Mn²⁺
 3. The regenerative fuel cell of claim 1, wherein the cathodecompartment comprises an additive comprising Ti(IV).
 4. The regenerativefuel cell of claim 1, wherein the cathode compartment comprisesTi(SO₄)₂.
 5. The regenerative fuel cell of claim 1, wherein the cathodecompartment comprises TiO²⁺ ions.
 6. The regenerative fuel cell of claim1, which includes at least one vessel configured to contain the liquidcatholyte containing the cathodic electrochemically active species,which first vessel is connected, in the power delivery mode, to thecatholyte compartment for delivering liquid catholyte containing theelectrochemically active species to the catholyte compartment.
 7. Theregenerative fuel cell of claim 6, wherein the at least one vessel isconnected, in the energy storage mode, to the catholyte compartment forreceiving catholyte containing generated electrochemically activespecies from the catholyte compartment.
 8. The regenerative fuel cell ofclaim 1, which includes at least one vessel configured to contain theliquid catholyte containing spent electrochemically active species,which second vessel is connected, in the power delivery mode, to aconduit for receiving the catholyte containing spent electrochemicallyactive species from the catholyte compartment.
 9. The regenerative fuelcell of claim 8, wherein said at least one vessel is connected, in theenergy storage mode, to a conduit for supplying the catholyte containingspent electrochemically active species to the catholyte compartment. 10.The regenerative fuel cell of claim 1, which includes a pressurized gassource vessel configured to contain hydrogen, which gas source isconnectable, in the power delivery mode, to the anode.
 11. Theregenerative fuel cell of claim 10, wherein the pressurised gas sourcevessel is connectable, in the energy storage mode, to the anode toreceive hydrogen generated in the energy storage mode.
 12. Theregenerative fuel cell of claim 11, which includes at least onecompressor configured to pressurise hydrogen generated at the anode inthe energy storage mode for storage in the pressurised gas sourcevessel, and optionally also a hydrogen expander-generator to deliverelectricity as a result of expansion of the compressed gas.
 13. Theregenerative fuel cell of claim 1, wherein the membrane is a protonexchange membrane.
 14. The regenerative fuel cell of claim 1, whereinthe membrane is porous to hydrogen ions and solvated hydrogen ions. 15.A method of operating a regenerative fuel cell in a) a power deliverymode in which it generates electrical power by the reaction ofelectrochemically active species at an anode and at a cathode and b) inan energy storage mode in which it consumes electrical power to generatesaid electrochemically active species, the cell comprising: a reversiblehydrogen gas anode, in an anode compartment; a reversible cathode in acathode compartment; a membrane separating the anode compartment fromthe cathode compartment, which membrane is capable of selectivelypassing protons; and wherein the method comprises, in said powerdelivery mode, carrying electrochemically active species to the anodeand to the cathode and, in a energy storage mode, carrying generatedelectrochemically active species away from the anode and away from thecathode wherein the redox reaction at the anode is:2H⁺+2e ⁻

H₂ (gas); the redox reaction at the cathode is selected from:V⁵⁺ +e ⁻

V⁴⁺,  (i)Ce⁴⁺ +e ⁻

Ce³⁺,  (ii)Mn³⁺ +e ⁻

Mn²⁺; and  (iii) the cathode compartment comprises an additivecomprising at least one of: (i) Ti(IV), (ii) Al(III), (iii) asurfactant, (iv) a chelating agent (v) a polymer, and (vi) a dendrimer.16. The method according to claim 15, wherein the redox reaction at thecathode is:Mn³⁺ +e ⁻

Mn²⁺
 17. The method according to claim 15, wherein the cathodecompartment comprises an additive comprising Ti(IV).
 18. The method asclaimed in claim 15, wherein the regenerative fuel cell is as claimed inclaim 1.